The fundamental event in prion formation involves a structural transition of a normal host encoded cellular protein PrP (PrPC) to a conformer rich in P-sheet, designated PrPsc. This conversion occurs by an unknown post-translational mechanism but the recent discovery of the doppel (Dpl) protein promises to help elucidate this process. DpI is a recently described PrP like molecule, which is encoded by a gene adjacent to PrP. We propose to investigate the structural requirements for prion propagation by the construction of a series of novel chimeric molecules containing regions of both PrP and DpI proteins. We have recently produced mice deficient for DpI that are viable and plan to study the effects of DpI deficiency on older mice and to determine whether or not DpI deficiency influences the course of prion infection. We have found that high levels DpI expression in transgenic (tg) mice in the CNS provoke degeneration of Purkinje cells as predicted from observations on different lines of PrnpO/O mice while low level expressors do not develop spontaneous neurodegeneration. Such mice will be used to explore the consequences of DpI expression in prion disease. Despite sequence homology of only -25% between DpI and PrP, NMR studies reveal that PrP and DpI are structurally quite similar. This provides an unusual opportunity to dissect some of the structural requirements for prion replication within the limits imposed by the small number of constructs that can be expressed and studied in Tg mice. Molecular modeling studies will help guide the construction of structurally sensible PrP:Dpl chimeras. These chimeras will be screened in normal and scrapie infected mouse neuroblastoma cell lines. Informative constructs will be then tested in vivo by construction of Tg mice. We plan to introduce our transgenes onto a genetic background based upon a new "clean" PrP knockout and to investigate of the extent of functional overlap between PrP and DpI by generating mice deficient for both genes.